1. Field of the Invention
The present invention relates to a reactive power compensation system and, particularly, to a reactive power compensation system used for preventing the fluctuation of power voltage caused by the variation of load current, which is mainly the reactive power, attributable to the irregular variation in the length of arc in a steel producing arc furnace.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing the conventional reactive power compensation system disclosed in article "Static Reactive Power Compensation System" in Mitsubishi Electric Corp. Technical Report, published in December 1984. In the figure, indicated by 1 is an a.c. power source, 2 is power source impedance, 3 is a variable load such as a steel producing arc furnace, 4 is a current transformer for detecting the load current, 5 is a voltage transformer for detecting the power voltage, 6 is a capacitor, 7 is a reactor, 8 is a thyristor switch for controlling the lagging reactive current flowing through the reactor 7, 9 is a reactive power compensating circuit constituted by the capacitor 6, reactor 7 and thyristor switch 8, and 10 is a control circuit for the reactive power compensating circuit 9.
FIG. 2 shows in detail the control circuit 10 in FIG. 1, where indicated by 11 is a reactive power calculating circuit, and 12 is a thyristor firing angle control circuit which determines the firing phase angle of the thyristor switch 8 in accordance with the calculated value of reactive power.
The operation of the above system arrangement is as follows: When the variable load 3 such as a steel producing arc furnace is connected to the power system of the a.c. power source 1, reactive power Q.sub.L attributable to the load 3 creates a voltage variation .DELTA.V at the power receiving point A as evaluated by the following equation. EQU .DELTA.V=X.sub.S .multidot.Q.sub.L ( 1)
where X.sub.S represents the value of power impedance 2 shown in FIG. 1.
In order to suppress the voltage variation .DELTA.V, the reactive power compensating circuit 9 made up of the capacitor 6, reactor 7 and thyristor switch 8 is employed. The reactive power compensating circuit 9 supplies compensating reactive power Q.sub.C back to the power system in proportion to the reactive power Q.sub.L flowing through the load 3 by determination of the thyristor firing angle by the thyristor firing angle control circuit 12 in the control circuit 10 on the basis of the reactive power signal calculated by the reactive power calculating circuit 11 in response to the voltage and current values detected by the current transformer 4 and voltage transformer 5. As a result of this reactive power feedback, the voltage variation at the receiving point A becomes: EQU .DELTA.V=X.sub.S (Q.sub.L -Q.sub.C) (2)
Accordingly, by making Q.sub.C substantially equal to Q.sub.L, the voltage variation .DELTA.V is diminished.
In the conventional reactive power compensation system, arranged as described above, the thyristor switch 8 controls the reactive power once a half power cycle at most. This results in a control retardation of a half power cycle at maximum, and the reactive power compensating circuit 9 is not sufficiently responsive to a fast varying reactive power. In consequence, when the load current includes harmonic components as in the case of a steel producing arc furnace, the voltage fluctuation can barely be reduced to about 1/3 at most, and a further reduction is left unsolved.